Field of the Invention
The invention concerns a method for controlling a magnetic resonance imaging apparatus as well as a control sequence-determining computer and a magnetic resonance imaging apparatus that implement such a method.
Description of the Prior Art
In medicine two-dimensional or three-dimensional image data as well as a time series of image data are often generated, using modern imaging methods, and such data are used for visualizing a mapped examination object. These imaging methods are also used for applications outside of medicine. Such imaging methods include, inter alia, magnetic resonance imaging, also called magnetic resonance tomography.
In magnetic resonance tomography, a static basic magnetic field Bo, for initial orientation and homogenization of magnetic dipoles to be examined, is usually overlaid with a rapidly-switched magnetic field, known as the gradient field, for spatial resolution of the imaging signal. In order to determine material properties of an examination object to be mapped, the dephasing or relaxation time is ascertained following a displacement of the magnetization from the initial orientation, so different relaxation mechanisms or relaxation times typical for the material can be identified. The displacement usually occurs due to a number of RF pulses, and the spatial resolution is based on a manipulation of the displaced magnetization, defined in terms of time, with the use of the gradient field in what is known as a scan sequence or control sequence. Such a scanner or control sequence defines an exact sequence over time of RF pulses, changes in the gradient field (due to emission of a switching sequence of gradient pulses) and the acquisition of scan (raw data) values. In addition to relaxation, there is also a range of further procedures for contrast imaging, such as, for example, flux measurement and diffusion imaging.
Typically there is an association between scanned magnetization—from which the material properties can be derived—and a coordinate of the scanned magnetization in the space in which the examination object is arranged, with the use of an intermediate step. In this intermediate step acquired magnetic resonance raw data are entered at respective readout points in a memory. The collection of such data in the memory is known as “k-space”, and the coordinates of data in k-space are coded as a function of the gradient field. The value of magnetization (in particular the transverse magnetization, determined in a plane transverse to the above-described basic magnetic field) at a specific location of the examination object can be ascertained from k-space data with the use of a Fourier transformation. In other words, k-space data (magnitude and phase) are needed in order to calculate a signal strength of the signal, and optionally its phase, in the position space (spatial domain).
Magnetic resonance tomography is a relatively slow type of imaging because the data are sequentially recorded along trajectories, such as, for example lines or spirals, in Fourier space (k-space). The method of recording images in two-dimensional slices is much less prone to errors compared to recording in three dimensions, because the number of coding steps is smaller than with a three-dimensional method. In many applications therefore, image volumes with stacks of two-dimensional slices are used instead of a single three-dimensional recording. However, the image recording times are very long due to the long relaxation times of the spins, and this entails a reduction in comfort of the patient being examined.
With a different type of scanning, selectively excited sub-volumes, known as “slabs”, are spatially encoded with the use of a three-dimensional scanning method.
In the case of a movement of the patient, the consistency of the scanned data is lost with the described imaging methods, and this is reflected by image artifacts in the reconstructed image. For this reason, it is desirable to compensate the movement of the patient. This is done either retrospectively (after the scan) or prospectively (as early as during the scan).
Methods of prospective movement correction correct the movement of the patient as early as during the performance of the scan. This occurs, for example by the field of view (FoV), which represents the section of the anatomy to be acquired, being tracked during the scan such that the anatomy contained therein does not change despite movement. A large number of methods exist in order to achieve this. For example, external systems, in particular cameras, are used for monitoring the position of the examination object. Monitoring can also be implemented with the use of markers that are visible to (detectable by the magnetic resonance scanner. Characteristic anatomical structures (landmarks) can also be observed instead of markers when monitoring. Furthermore, additional image-based navigators are also used to detect a change in the position of the examination object and to track the field of view accordingly. In the case of time-resolved image data, the image data itself can also be used for detecting a change in position; an additional acquisition of image-based navigators can optionally be omitted in this case therefore.
When using image-based navigators, the imaging MR sequence is used to depict the anatomy in navigator images at successive instants. A reference volume is typically scanned at the beginning of the scan and subsequent navigator images are registered at this reference time. The detected movement is then returned to the MR pulse sequence and the field of view or the image region is tracked by this movement for the next partial acquisition, and the movement is thereby compensated as early as during the scan.
A slice-based acceleration technology has become established in recent years to accelerate the image recording, or to accelerate the required scan duration for acquisition of an individual image volume. Using this technology known by the names “Simultaneous Multi-Slice” (abbreviated to SMS or SAMS), “Slice Acceleration” or even “Multiband”, multiple slices are simultaneously excited and read out (as described e.g. Breuer et al. Magnetic Resonance in Medicine 53:684 (2005), Souza et al. Journal of Computer Assisted Tomography 12:1026 (1988), Larkman et al. Journal of Magnetic Resonance Imaging 13:313 (2001). For example, with an acceleration factor of three, three slices respectively are simultaneously excited and read out. This reduces the required repetition time TR (the time until successive pulse sequences are applied to the same slice) to one third of the required time. Accordingly, the time necessary for acquisition of a volume can be reduced to a third of the required time, for example in the case of functional imaging (fMRI, BOLD) or diffusion imaging by echo-planar recording. The reduction in the scan time and the improvement in the scan rate in terms of time are cited as the main advantage of these methods in the literature.
Even when scanning selectively excited sub-volumes, it is possible to scan a number of sub-volumes simultaneously with the use of “multi-slab” imaging in order to accelerate the recording process. A procedure of this kind can be regarded as an intermediate stage between 2D-multi-slice imaging and complete 3D imaging.
Conventional methods for movement correction can be divided into two groups. The first group includes methods in which the image data generated directly by the MR pulse sequence is used for determining the movement. These methods are used for sequences that generate a 4D time series which contains the identical anatomy in each case. One example of this is functional, echo-planar imaging as proposed in Thesen, Magnetic Resonance in Medicine 44.3:457, and in a dissertation by Thesen “Retrospektive and prospektive Verfahren zur bildbasierten Korrektur von Patientenkopfbewegungen bei neurofunktioneller Magnetresonanztomographie in Echtzeit” (2001) Retrospective and prospective methods for image-base correction of patient head movements in neurofunctional magnetic resonance tomography in real time.
The second group includes methods with which the MR pulse sequence is changed such that, in addition to the intended MR image data, navigator images are also generated. The navigator images are then used to compensate the patient movement after or even during the scan.
The two outlined approaches for prospective or retrospective movement correction both have specific drawbacks. For the first group of methods, only the exactly identical image data is available for movement correction. This can be disadvantageous, for example, if the image contrast or the noise behavior is not suitable for movement detection. In addition, it is possible that the mapped anatomy is not suitable for the chosen method of movement correction. This is the case, for example, for the assumption of a rigid movement model if non-rigid movable parts of the anatomy are mapped (e.g. the movement of the eyeballs or of the jaw when scanning the head) or the contained structures of specific, non-rigid artifacts are affected (e.g. non-rigid distortions due to inhomogeneities of the magnetic field).
The second group of methods requires an additional scan of an image-based navigator. With some sequences this can be inserted in unused periods of the sequence. Tisdall et al. “International Society for Magnetic Resonance Medicine 2009, “MPRAGE using EPI navigators for prospective motion correction” propose this approach in order to insert an additional 3D-EPI navigator in the inversion time in an MPRAGE sequence. In general, however, this is not possible for all sequences without changing the time response, in particular it is not possible for those sequences that do not have the unused periods necessary for this.
An additional drawback of the described methods (in particular of the second group) is that the recording of the navigator image and the acquisition of the MR signal for imaging take place with at slightly staggered intervals. Even for the case where the movement of the navigator can be exactly detected and be compensated by the sequence, resulting artifacts remain in the MR data because the movement content between the time of acquisition of the navigator and the time of acquisition of the desired MR imaging signal is not detected and therefore cannot be compensated either.
A further problem is that the navigator images, if they are acquired using an MR pulse sequence that differs from the MR imaging sequence, can have different image artifacts, such as distortions. If these artifacts are offset by the movement detection and compensation, artifacts can be generated in the MR imaging sequence that, without correction, would not be included at all.